One problem with “forced copulation” a.k.a. “rape” is the possibility of causing sufficient injury to the female she is unable to complete her part of the reproductive cycle and deposit eggs/give birth. Male mallard ducks, for instance, have been known to occasionally gang rape a female to the point she drowns. This does not improve reproductive success for anyone involved.
In some species the females are able to hold their own against males. I once saw a mare kick a wannabe stud in the genitals. The male horse almost fell down, was clearly in severe pain, and was only able to hobble away. (No, he didn’t try again). In many species of birds the female is bigger than the male. In raptors often notably so, such that the male is likely in more danger from the female than vice versa. On the other hand, bedbugs have a crazy system where it is, apparently, for the males to skip the vagina-equivalent and stab the females with their penis to reproduce, a “technique” called “traumatic insemination”. Biology can be crazy.
Anyhow - as bedbugs demonstrate this can be a strategy in a species but examining the rest of the animal kingdom shows it’s pretty rare as an exclusive method of making more of the same.
Reasons rape may not be effective at making more of a particular critter:
As mentioned - possibly of injuring the female to the point she doesn’t reproduce the rapist’s offspring, or is even killed (in which case she really won’t reproduce).
The MALE might be injured, perhaps to the point of impairing future reproductive ability. Pretty sure the porcupine species is rape free for obvious reasons. I can not imagine a male black widow spider trying “sexual assault” - not only is the female significantly larger, there’s a good chance she’s going to eat him anyway. If he leads with force he’s definitely going to be dinner. Female hawks tend to be about 1/3 bigger than the males and proportionately stronger, it’s pretty unlikely he’d try it and she could easily kill him if he pissed her off enough.
Some critters - female mallards, for example - have internal means that hamper insemination unless the female is willing and cooperative. Which is unlikely if she’s being forced.
Rape might occur when the female isn’t actually ready to conceive, before or after that narrow window when she might be more cooperative.
Rejection of the rapist’s offspring - this would seem to require a knowledge of how this biology works which, we assume, is limited to people but hey, we’re critters to. While there are women who keep children that are the product of rape others reject them, perhaps forcefully, which would work against rape as a reproductive strategy in humans. Wild animals sometimes reject offspring for no reason we can see so maybe that’s a reason, how would we ever know? (Really, we can only know this about humans)
But, beyond that - animals where hundreds or thousands of offspring are created and sent out into the world have an incentive to force conception because… why? They’re already producing gajillions of young, why risk anything at all to do more?
You tend to see rape where the chances for males successfully mating/reproducing are low, instances where risking the downsides listed above are worth more than never reproducing at all. I would think this would make rape more likely among the K-species where reproductive chances are limited and thus each is more valuable. r-Species where pretty much all males get a chance, particularly those where conception occurs outside the body (frogs and most fish, for example), it would be either less useful or even pointless.